The three isoforms of transforming growth factor-beta have previously been implicated in embryonic development of the heart as well as in repair of myocardial damage following ischemia/reperfusion injury. TGF-Beta 1 has been localized intracellularly to mitochondria and contractile filaments of cardiac myocytes, suggesting an intrinsic role in the myocardium. To attempt to identify the mechanisms of cardioprotection, we have undertaken new studies based on examination of the effects of TGF-Beta and of inflammatory mediators such as interleukin-1, tumor necrosis factor, interferon gamma, as well as endotoxin, on cultured neonatal cardiac myocytes. We have shown that exogenous TGF-Beta stabilizes the beating rate of these cardiac myocytes cultured on fibroblast matrix, and sustains their spontaneous rhythmic beating in serum-free medium. Moreover, using blocking antibodies to TGF-Beta, we have shown that endogenous TGF-Beta secreted by these myocytes acts in an autocrine fashion to maintain their beating rate. In contrast, interleukin-1 Beta, an inflammatory mediator secreted by immune cells during myocardial injury, inhibits the beating of cardiac myocytes, and TGF-Beta can overcome this inhibition. The antagonistic effects of TGF-Beta and IL-1 were not observed when the myocytes were cultured on gelatin, as compared to native fibroblast matrix. The data indicate that TGF-Beta is an important regulator of contractile function of the heart and have significant implications for understanding cardiac physiology in both health and disease.